Not Applicable
Not Applicable
The present invention relates to the transport of packet traffic in optical communications networks employing synchronous signaling techniques, such as networks employing the Synchronous Optical Network (SONET) signaling format.
There exist today a large number of wavelength-division multiplexed (WDM) point-to-point transmission systems and WDM networks. Many of these systems have been designed to support a fixed number of wavelengths and a predetermined data format. SONET is a common framing format for data transport in long-haul and metropolitan-area WDM carrier networks. The SONET frame provides a convenient standard mechanism to multiplex and transport circuit-switched traffic in high-speed backbones. There are a number of readily-available standard components for SONET-based systems, such as clock recovery units designed for SONET rates, SONET framers and multiplexers, so that the task of network design is simplified. SONET also provides mechanisms to support network functions such as error detection, alarm insertion, automatic protection switching, etc. The common frame format also allows for advanced functions, such as electro-optic switching, signal regeneration, internal signaling, and signal restoration, that are independent of the format of the native traffic carried by the network. Because of these and other beneficial characteristics, SONET framing is used in most WDM systems deployed today.
In contrast, many packet-switched local area networks (LANs) use framing defined in the long-established Ethernet standard. Unlike SONET, Ethernet and other LAN protocols rely on non-synchronous signaling techniques. While the original Ethernet standard contemplated the use of copper coaxial cable as a physical transmission medium, there have been recent extensions, such as Gigabit Ethernet (GbE), that contemplate the use of optical media. GbE is an evolution of the Ethernet LAN standard to gigabit rates. It uses the same frame format specified by the original Ethernet standard, and the same multiple-access protocol and flow-control methods as the 10 Mb/s and 100 Mb/s Ethernet standards. GbE also employs the same variable frame length (64-1514 byte packets) specified in the Ethernet and Fast Ethernet standards. This backward compatibility makes it easier to connect existing lower-speed Ethernet devices to GbE devices using LAN switches and routers for speed adaptation.
The GbE standard defines both the physical (PHY) and the medium access control (MAC) layers of the OSI architecture. The PHY layer deals with the transmission of bits over physical channels. In particular, the GbE standard specifies a raw bit rate of 1.25 Gb/s. In full-duplex mode, GbE employs a packet switch at the network hub that operates without any collisions. While operating in half-duplex mode, GbE uses the classical CSMA/CD mechanism to resolve contentions for access to the shared physical transmission medium, although the slot duration has been extended over that of Ethernet and Fast Ethernet to allow transmitters sufficient time for collision detection.
Many of the PHY layer features of GbE also appear in another standard called Fiber Channel (FC), which is a protocol for point-to-point high-speed computer interconnection. The FC protocol precedes GbE by several years and has become very popular in practice. FC defines a PHY layer protocol for the point-to-point interconnection of two nodes at a rate of 1.0625 Gb/s.
The FC standard specifies a line encoding algorithm known as xe2x80x9c8b/10bxe2x80x9d encoding to achieve DC balance and xe2x80x9crun length limitingxe2x80x9d, i.e., providing a minimum rate of signaling transitions in the data stream to ensure adequate clock recovery at a receiver. The 8b/10b code converts a byte-wide data stream of random xe2x80x980xe2x80x99s and xe2x80x981xe2x80x99s into a DC-balanced binary stream with a maximum run length of five xe2x80x981xe2x80x99s or five xe2x80x980xe2x80x99s. This is accomplished by converting 8-bit blocks of the non-encoded data stream into 10-bit code words selected to achieve the run-length limitation. The encoding and decoding algorithms can be readily implemented. At the encoder, two different sub-codes operate on 5-bit and 3-bit sub-blocks of each input block to yield corresponding 6-bit and 4-bit sub-codewords, with minimal interaction between the 5b/6b and the 3b/4b sub-codewords. The code constrains the disparity between the number of xe2x80x981xe2x80x99s and xe2x80x980xe2x80x99s in a sub-block to be xe2x88x922, 0, or +2. Furthermore, by assigning two sequences to certain inputs, and by switching between these two sequences according to the state of the encoder, the encoder limits the running disparity observed at sub-block boundaries to +1, 0, or xe2x88x921. Thus, the 8b/10b code provides a mechanism for efficient clock recovery and DC balance, albeit at the expense of a 25% increase in the raw signaling rate of the channel.
Because SONET and GbE have been separately optimized for transport and data networking, respectively, the existing art has treated these signaling mechanisms in an isolated manner. Thus, a typical WDM network with OC-48 interfaces (a particular type of SONET signal) can accept only SONET-framed traffic at the OC-48 rate (2.488 Gb/s); there is no ready mechanism for accepting variable-sized packets transmitted at a non-SONET rate, such as the 1.25 Gb/s rate of a GbE network. While it is possible to use a device known as a xe2x80x9cSONET framerxe2x80x9d to translate a GbE signal into an OC-48 SONET signal, this approach would result in poor utilization (about 40%) of the available bandwidth at each wavelength transporting such traffic.
An alternative optical transport strategy is to use a so-called xe2x80x9ctransparent interfacexe2x80x9d, which foregoes SONET framing and performs a simple frequency translation at the optical network boundary. The advantage of this approach is bit rate and format independence. However, this approach suffers from drawbacks such as the lack of performance monitoring within the optical network; the inability to perform opto-electronic circuit switching; and increased jitter accumulation in regenerator cascades.
There is a need for an optical network interface that can accept and multiplex multiple GbE/FC signals into a synchronous format signal such as a SONET signal, in order to provide for multiplexing, adding/dropping, and monitoring LAN traffic in an optical backbone.
In accordance with the present invention, methods and apparatus are disclosed to perform multiplexing and transport of Gigabit Ethernet (GbE) and Fiber Channel (FC) signals on WDM networks using SONET signaling. A line interface in an optical communications node is described that accepts multiple GbE/FC streams, processes these individual streams to reduce their data rates, and combines the rate-matched stream into a standard-rate SONET stream which will be transported on a wave. The data-carrying capacity of the SONET network is efficiently utilized, and important functionality such as synchronization and performance monitoring available in SONET systems is provided.
A transmitter decodes encoded data signals such as GbE or FC signals to generate a corresponding plurality of non-encoded data signals, so that the sum of the respective signaling rates of the non-encoded data signals is no greater than the data-carrying capacity of the communications link. In one embodiment this decoding reduces the signaling rate of each signal to about 9/10 the signaling rate of the encoded signal. The non-encoded data signals are multiplexed together to form a multiplexed data signal, the multiplexed data signal is transmitted over the communications link. In one embodiment the multiplexed signal is an OC-48 or similar synchronous transport signal. A receiver receives the multiplexed data signal from the communications link, de-multiplexes the received signal to obtain the non-encoded data signals, and re-encodes each de-multiplexed data signal to re-create the encoded data signals at the receive end.
To enable correct de-multiplexing at the receive end, the disclosed system employs packetization of the signals after decoding, where each packet includes an address or stream identifier. The transmitter converts sequential data blocks of each non-encoded signal into corresponding packets, and asynchronously interleaves the packets of the non-encoded signals to create the multiplexed data signal. The packets may be of a fixed size, which is chosen to achieve a desired balance between link bandwidth efficiency and storage efficiency. In one embodiment a 67 byte packet size is used. The receiver uses the stream identifier of the packets to separate the streams for the different signals, and then re-encodes each signal to re-create the encoded signals at the receive end.
The stripping of coding overhead at a transmitter and re-encoding at the receiver advantageously permits multiple signals, such as multiple GbE or FC signals, to be carried on a single transport signal such as an OC-48 signal. All the advantages of SONET transport, such as error monitoring, flexible add/drop, etc., can be realized for the GbE or FC traffic. Moreover, because SONET includes a mechanism for maintaining synchronization, accurate end-to-end operation is obtained despite the removal of the timing information embedded from the encoded data signals before transport over the communications link.
Other aspects, features, and advantages of the present invention are disclosed in the detailed description that follows.